1. Field of the Invention
This invention relates to a recording apparatus employing a toner image produced on an electrostatic latent image forming medium, such as electrophotographic printing apparatus, and particularly relates to a method of detecting the end of useful life of the image forming media, as well as the developer carrier.
2. Description of the Related Art
In an apparatus utilizing toner development, such as an electro-photographic apparatus, the toner is consumed for development. Therefore, it is required to supply toner into a developing unit so as to keep the toner density constant.
In order to control this toner density, there has been a method employing a particular toner mark formed on a latent image forming medium, i.e. on a photosensitive drum, and an optical detector which is composed of a light emitting device to project a light spot on the surface of the photosensitive drum and a light receiving device to receive light reflected from the projected spot on the photosensitive drum. The optical detector detects the toner mark so as to measure its toner density. According to this measured toner density, a supply of the toner to developer in a developing unit is controlled so that a uniform toner darkness is achieved in the printed image.
The optical detector is employed also for detecting whether a developer (carrier) in the developing unit is worn out having reached the end of its useful life, as well as for detecting the end of the photosensitive drum's useful life caused by contamination, such as the generation of filming, etc. When the carrier is fatigued, the toner clings to the carrier causing an increase in electrical resistivity of the carrier. Accordingly, an undesirable toner stays on the photosensitive drum where no toner should remain. This undesirable toner deteriorates the contrast of printed images. When the photosensitive drum is fatigued worn out, the photosensitive drum loses its electric charging capability. Undesirable toner remains on the area to which no light has been projected. As a result the contrast is also deteriorated. Thus decreased reflection from the drum surface causes a decrease in the received light level as well as in an amplitude of the toner mark in the received light.
The optical detector must be located close to the photosensitive drum in order to detect the toner mark and the contamination of the drum itself. As such the light emitting window as well as the light-receiving window of the detector is likely to be contaminated with the toner floating out from the photosensitive drum. When the window of the optical detector is contaminated, the light receiving level of the detector decreases thereby resulting in greater error in detecting the toner density, etc..
When the optical detector is contaminated, the detector must be cleaned up. Conventionally, air has been blown onto the detector in order to remove the toner contamination on the detector. However, the air blowing method has a problem because a narrow gap between the photosensitive drum and the detector causes difficulty in blowing the air directly onto the light receiving surface. Furthermore, there is also a problem in that strong blowing disturbs the toner image on the photosensitive drum.
When the level of the received light reflected from the photosensitive drum, except from the toner mark, deteriorates to a certain level, it is determined that the detector is contaminated, and then cleaning must be performed. However, in this method has a problem in that not only is a degree of the contamination not correctly grasped, but also a fast contamination caused from many print operations shortens the interval of the cleaning operations, resulting in an increase of maintenance.
Furthermore, even though the deterioration of the drum or the carrier takes place much slower than the detector's contamination once the drum or the carrier is deteriorated, it is impossible to determine whether the decline of the received light level is caused by the detector's contamination, by the deterioration of the photosensitive drum or by the deterioration of the toner carrier. Accordingly, it is impossible to properly compensate the variation of the toner density.
Furthermore, there has been proposed a method to detect the toner density by a differential output of a pair of detectors as disclosed in Japanese Patent Publication, No. Sho 63-14348, etc.. In this method of providing a pair of the detectors, it is a problem that a detector is required, which further requires an additional space and cost for installation. It is also a problem that an alignment of the optical detector to the toner mark is difficult. It is impossible to determine whether the deterioration of the received light level is caused by the detector's contamination, from by the deterioration of the photosensitive drum or by the deterioration of the toner carrier.